1. Field of the Invention
This invention relates to improvements in logic circuits or the like, and more particularly to improvements in circuits and methods for producing a known output logic state at insufficient supply
2. Relevant Background
In the design of typical logic cells, commonly an inverter circuit that includes two transistors is found. One transistor is generally connected between an output node and a supply rail, and the other transistor is generally connected between the output node and a reference supply rail, or ground. The control elements of the transistors receive an input, or control, voltage. In CMOS circuitry, for example, a typical inverter logic circuit which may be found has a p channel MOS (PMOS) transistor connected in series with an n channel MOS (NMOS) transistor between a supply voltage and ground, or a reference potential.
When the input voltage is low, for instance, the transistor connected to the supply rail is turned on, thereby conducting the supply voltage to the output node. On the other hand, when the input voltage is high, the other transistor is turned on, to connect the output node to the reference potential, or ground. As logic input signal is applied simultaneously to the gates of both transistors, the output is derived from their common drains.
Typically, such transistors are designed with low voltage thresholds in order to meet the desired circuit specifications. Both transistors are normally designed to have a V.sub.t that is sufficiently less than the supply voltage required for the output to be in a desired state during normal operation. In the construction or fabrication of such semiconductor logic circuits, however, it is often desirable that the logic circuit provide a known output when the power supply voltage is within a predetermined, typically insufficient, voltage range. For example, on power up of a circuit, as the power supply voltage begins to rise, the output of the circuit may be indeterminate until the voltage reaches a predetermined threshold. If the circuit were to be used, for instance, to provide a reset signal to downstream circuits, an indeterminate output state might allow the downstream circuits themselves to assume unknown or indeterminate states. This is, of course, undesirable. Thus, in such case, generally it is desirable to force the output of the circuit to a known output state, at such insufficient supply voltages, rather than allow the circuit to exhibit such possible indeterminate state.